The present invention relates to a process for preparation of fructose-containing solid sugar and particularly to a technique for dehydration and solidification of fructose-containing liquid sugar. It should be realized that, in this specification, the term "solidification" includes both crystallization and formation of amorphous powder.
Fructose is obtained by hydrolysis (inversion) of sucrose or isomerization of glucose so far as it is on an industrial scale. In these cases, fructose-containing liquid sugar of two types primarily obtained are referred to as invert sugar and isomerized sugar. Assumed that said FE (Fructose Equivalent) is defined as ##EQU1## the invert sugar will have a FE of substantially 50 and the isomerized sugar will have a FE of substantially 40. Although said invert sugar or isomerized sugar is commercially available under such state, as a recent trend, these are available also in the form of secondary product obtained by a separation treatment through use of ion-exchange resin or inorgnic adsorbent so that the FE is increased substantially to 90, in view of a high value added of fructose.
Said fructose-containing sugar is liquid of which the moisture content is 10 to 20% by weight and has been directly used as food sweetener in practice mainly due to a difficulty for isolation of fructose from the fructose-containing liquid sugar as well as a difficulty for crystallization of fructose which is, in turn, due to a particular nature thereof. Fructose takes different molecular structures depending on whether it is crystal or solution in which fructose is present. Namely, it takes .beta.-pyranose structure in crystal while it takes .alpha.-pyranose structure together with a noticeable quantity of furanose structure and normal chain molecules, although the equilibrium of these structures or molecules of different types has not still been theoretically backed up. One of the most important physical properties of fructose is its solubility to water substantially higher than values exhibited by the other natural sugars and specifically as high as 96 W/V%. This means that fructose has a high affinity to water. However, decomposition temperature of fructose lies at a relatively low level between 102.degree. C. and 104.degree. C. and therefore is easily denatured by heating, so that it is impossible to remove water contained therein simply by heating. This makes it further difficult to solidify fructose. Effective solidification, i.e., crystallization or formation of amorphous powder of fructose having such characteristics requires a technical consideration higher than for the other sugars and, in fact, various techniques have already been attempted. Although there have been commercially available pure fructose crystal obtained by crystallizing the liquid fructose isolated as precisely as to FE value from 98 to 100 mainly as medical use, the well known process for pure fructose crystallization has various disadvantages. Namely, in the process of well known art, a quantity of alcohol at least same as and at most twice the quantity of moisture containing liquid fructose has been added to and dissolved in the latter, then subjected to seed crystal addition treatment for aging and thereafter to solid/liquid separation. According to this process of prior art, a filtrate obtained consists of hydrate alcohol and fructose dissolved in this alcohol and therefore the recovery of said alcohol as well as treatment of residual moisture containing fructose after said alcohol recovery are left to be done. It can not be expected to achieve crystal separation by directly recrystallization of said residual moisture containing fructose and a possible yield will be extremely low, so that a dehydration is required simultaneously with a purification by ion-exchange resin. Such process of prior art has been disadvantageous in a high loss of alcohol, fructose yield for every operation as low as 40 to 70% by weight, and in a batch or semi-batch operation. These inconveniences have prevented a production from being performed at an industrial or commercial scale.
Although a process for preparation of pure crystal fructose has been described above as the process of prior art, a technique for solidification of fructose-containing liquid sugar in the order of FE 40 to 90 has recently been interested more and more for a wide prospective market as a demand for fructose, particularly as sweetener of refreshing beverage has recently increased. However, solidification of fructose-containing liquid sugar having its FE lower than 100 is more difficult than that of pure liquid fructose and a more effective method of solidification has been demanded. Fructose-containing liquid sugar of FE lower than 100, for example, of FE 90 is necessarily a multi-ingredient system which is difficult to be crystallized. When usual process of crystallization in non-aqueous solvent is employed, such multi-ingredient system would produce crystalline and amorphous solids in the form of the finest particles and increase its viscosity due to mixing with molasses, resulting in that a desired solid/liquid separation becomes extremely difficult or impossible. Accordingly, this process is most disadvantageous in that a relatively simple filtration treatment using a filter can not be employed here. The technical interest has been concentrated into a process for preparation of fructose-containing solid sugar from fructose-containing liquid sugar, particularly liquid sugar having its FE in the order of 90, which has a high value added, or liquid sugar of FE in the order of 40 which can be obtained at a lower cost. A principal object of the present invention is to provide such process.